Did the Earth move for you? Well it did for me while I took 30s exposures of Orion from Stuart’s bedroom desk with a regular 50mm lens and my Canon DSLR mounted on a very simple, non-tracking, tripod. Actually I was just practicing with Robbie’s remote control app on my mobile phone, DSLR Controller – I didn’t even open Stuart’s bedroom window. I used the phone to set the camera to shoot ‘continuously’ and left it to accumulate multiple 30s images. Then I fitted the lens cap and let it take a few more ‘dark frames’, which I can use to reduce the image ‘noise’ caused by setting the camera sensor to represent a very high ‘film speed’. Unfortunately the results were spoiled because I forgot to set the lens aperture wide open, and I left it at f11 – fine for daytime photography last time I used it! Nevertheless the results led to an interesting exercise with some post-processing software called Focus Magic.
In ordinary photography if the subject moves or the camera shakes you can just try again, and say ‘hold still’. That doesn’t work with astrophotography because the the earth ignores you and continues to turn (at about 360° per day) while the stars remain fairly fixed. There is a ‘rule of 600’ which let’s us guesstimate how long an exposure we can get away with before motion blur leads to unacceptable star trails – limit the exposure to 600/focal length of the lens (in mm). So with my 50mm lens I should be OK with exposures up to 600/50 i.e. 12s. Very often though there’s not enough starlight for an ordinary lens and a normal ‘film speed’ (e.g. ISO 400) to get a decent image with the rule of 600. As ever, we have to compromise between exposure time, lens aperture, and ‘film speed’.
Next time I’ll remember to open the lens wide, to f1.8, and maybe I’ll capture a few more background stars. Incidentally, did you spot the wee pinkish dot in Orion’s sword, that’s M42 of course, the Great Orion Nebula, without the aid of a telescope.